1. Field of the Invention
The present invention relates to bias circuits for radio frequency avalanche diodes.
2. Description of the Prior Art
Amplifiers and oscillators utilizing biased avalanche devices, operate on the principle that an avalanche device exhibits a negative resistance at a desired microwave operating frequency. Typical microwave operating frequencies are in the order of 1.5 GH.sub.z. However, avalanche devices typically also generate spurious submicrowave oscillations.
Avalanche devices typically exhibit negative resistances at undesired sub-microwave frequencies, for example, at frequencies in the order of 100 KH.sub.z to 500 MH.sub.z, as well as at the desired microwave frequency. If the impedance of the biasing network to the avalanche device presents an impedance match to the negative resistance at the low frequencies, spurious oscillations (hereinafter referred to as sub-microwave negative resistance oscillations) at such frequencies occur.
In addition, spurious relaxation-type oscillations, at frequencies (typically 50 KH.sub.z to 500 KH.sub.z) dependent on the voltage swing across the avalanche device and the RC time constant of the circuit, often occur when the bias circuit impedance will support such an oscillation.
The bias signal applied to avalanche devices, and in particular to high voltage swing devices, such as TRAPATT diodes, has typically been of a pulse nature to provide for power dissipation. The output signal of the device is accordingly of a similar pulse nature. Such bias pulses are typically generated by a high internal resistance constant current source driven by a pulse generator. The high internal resistance of the current source, typically in the order of 100K.OMEGA., typically controls the resistance of the bias circuit. In previous practice, particularly with large voltage swing devices, the bias pulses have been in the order of one-half microsecond, much shorter than the time for one-half a cycle of the spurious sub-microwave oscillations. Accordingly, the effects of such spurious oscillations have not been apparent in typical prior art circuits utilizing large voltage swing devices. However, in some applications, for example in phased array radar, it is desirable to increase the pulse width of the bias signal to, for example, 50 microseconds or longer. Such pulse widths are longer than one-half cycle of the spurious oscillations, and accordingly, the effects of the spurious signals are manifested, typically rendering the system inoperable, and in some instances causing the avalanche device to burn out.
The problem of spurious sub-microwave negative resistance oscillations in low voltage swing IMPATT oscillators has been addressed in U.S. Pat. No. 3,792,375 issued Feb. 12, 1974 to C. A. Brackett. The Brackett oscillators utilize a resistive network coupled in series between the bias line and the IMPATT diode. Such an arrangement, however, as will be explained below, would be ineffective in a circuit utilizing a high voltage swing avalanche device, such as, for example, a TRAPATT diode, which, as will be also explained, generates spurious relaxation oscillations at frequencies much lower than those generated in low voltage swing circuits.